The present invention generally relates to a dry etching method. In particular, the invention concerns a plasma etching method and a reactive ion beam etching method which are advantageously suited for etching silicon.
In hitherto known dry etching methods, CF.sub.4 -gas is most usually employed. As the consequence, carbon and CF.sub.n -polymers tend to be unwantedly deposited on the surface of a Si-wafer under etching. Another disadvantage is seen in that the etching rate is as low as of the order of 1000 A/min. More specifically, in the manufacture of semiconductor elements, CF.sub.4 l-gas has heretofore been used as a source gas in the process of dry-etching the substrate of Si, SiO.sub.2 or the like. Upon etching, there are generated various ions in the CF.sub.4 -gas plasma. When ions C.sup.+ and CF.sup.+ among the generated ions impinge on the Si-wafer at an energy level in the ranges of 100-2200 eV and 100-700 eV, respectively, deposition of these substances take place on the surface of the Si-wafer, as is illustrated in FIG. 1, in which a broken curve represents behavior of the C.sup.+ ion while a solid curve represents that of the CF.sup.+ ion. In the case of the plasma etching process, it is mainly those ions whose energy lies in a range of 100 to 600 eV that impinge onto the workpiece, i.e. a semiconductor wafer. Consequently, in the etching process using only CF.sub.4 as the discharge gas, it is impossible to attain a satisfactorily high etching rate due to the deposition. Under the circumstances, a small amount of O.sub.2 -gas or H.sub.2 -gas is usually added for the purpose of removing the deposition formed on the wafer surface.
By the way, in the manufacture of semiconductor elements, the etching process is often carried out with a photoresist pattern being used as a mask for the etching. Accordingly, when the amount of O.sub.2 is increased, the photoresist pattern may undesirably be etched away. Further, the addition of O.sub.2 and/or H.sub.2 in a small amount makes no contribution to the solution of the problem of the low etching rate.
Dry-etching using PCl.sub.3 was proposed and is known to exhibit anisotropic etching which depends on crystal orientation. Further the problem of deposition exists.